The central idea is that the billet index is hidden within the deck by attaching the billets to the backs of the relevant cards. The earliest record of this idea may be Rupert H. Slater's “Prediction” in Goldston's Magical Quarterly, Spring 1937, p. 255. Slater used double cards that formed pockets. These pockets held the appropriate prediction billets and allowed access to them from from the backs of the cards.
Three years later, Robert Brethen contributed “As In a Mirror Darkly” to The Jinx, No. 76, Jan. 1940, p. 501. Brethen's approach, dated to 1930 in the description, was simply to use a dot of paste to tack folded cigarette papers to the backs of the cards. Also see Max Abrams's Annemann: The Life and Times of a Legend, 1992, p. 425.